It is known to utilize an electrical cable to supply electrical energy to a downhole motor which drives a pump for producing oil or water from a well. In addition, U.S. Pat. Nos. 4,346,256 and 4,665,281 disclose the use of insulated electrical conductors enclosed in a metallic tube for supplying electrical power to a well pump.
However, the prior art has not recognized or has been directed to the effect that tensile loads and high temperatures will have on the relative motion of the inner electrical conductors to the outer metallic tube. Insulation and jacket materials allow higher modulus materials, such as copper or aluminum, to easily elongate or even yield the insulation, such as elastomers. This condition is exacerbated over the longer lengths typically encountered in water and oilwells. The primary failure mechanism in electromechanical well cables is conductor "z-kinking" whereby the electrical conductors will twist radially leading to electrical failure. Another term for z-kinking is called birdcaging and is defined as the permanent deflection of a wire rope forced into compression. The cause of z-kinking in electromechanical cables exposed to tensile and compressive forces and elevated temperatures stem from the high coefficient of thermal expansion of the electrical conductors (typically copper or aluminum) versus the tensile supporting member (typically steel) which leads to compressive loading of the conductors.
The present invention is directed to a solution to this problem by controlling the elongation of the metal components of the electrical cable to allow optimum performance under tensile load and at elevated temperatures.